One means for collecting solar energy is a solar pond. A solar pond may be defined as "a shallow artificial black bottom pond or lake in which the incident solar insolation is converted into a local temperature rise in the water near the bottom." (Mehta, G. D., "Non-Convecting Solar Ponds", Technical Report ETG-4, Hydronautics, Inc., Oct., 1975). A successful solar pond should develop simultaneously a high temperature at the bottom of the pond as a result of the radiation and a low temperature at the top. The low temperature is desirable to minimize excessive energy losses through evaporation, conduction and radiation.
The objective of high temperature at the bottom and low temperature at the top cannot be readily achieved, if at all, by using a pond containing water alone. With water alone, the higher temperature at the bottom of the pond decreases the density of the liquid relative to that at the surface, thus causing convection currents which quickly equalize the water temperatures. These density convection currents can be eliminated by introducing a density gradient maintained by a suitable salt concentration gradient. Such ponds are referred to as non-convecting solar ponds. They have been tested and studied rather extensively.
One of the advantages of the non-convecting solar pond is that it has a substantial storage capacity. It has been estimated that the solar pond could have a storage capacity of as much as 31 days, i.e. deliver substantial thermal energy for this period without incident sunlight.
The advantage of the storage capacity plus other advantages, such as low cost, make the non-convecting solar pond a promising candidate for the collection of unfocussed solar energy. On the other hand, there are certain disadvantages at present, the major one of which is maintaining the salt concentration gradient necessary for pond stability. The very existence of a salt concentration gradient causes salt diffusion which tends to destroy the gradient. The salt diffusion occurs because the brine is unsaturated at all levels with the salts usually used, such as NaCl or MgCl.sub.2, for which solubility is relatively insensitive to temperature. With such salts the saturation concentration is relatively constant over the substantial temperature range which usually prevails from the top to the bottom of the pond. Therefore, the brine is at a lower concentration relative to saturation at the upper, cooler level than at the lower, warmer levels, and diffusion of salt can proceed unimpeded. It has been suggested (Styris, D. L., et al, "The Non-Convecting Solar Pond--an Overview of Technological Status and Possible Applications", Battelle Pacific Northwest Laboratories, Report BNWL-1891-UC-13, Jan., 1975), that the problem of diffusion could be largely solved if the pond were substantially saturated with a salt having a solubility which is a direct function of temperature. In such a saturated non-convecting solar pond (saturated pond for short), one way of viewing the resultant action is that the salt could no longer diffuse successfully to a less concentrated (cooler) region because it would move to an already-saturated region, causing it to precipitate and sink to the hotter, now unsaturated region where it would redissolve. The saturated pond should be self-generating (assuming some temperature gradient always exists from the extra solar radiation absorbed near the bottom), self-maintaining, and self-repairing, all qualities which unsaturated ponds do not possess. Thus the saturated pond should be simpler in construction and operation than the unsaturated pond. Unfortunately, no saturated solar pond has been built because of the apparent lack of a temperature-sensitive solute that is cheap, stable, nontoxic, transparent, available in large quantities and the average solubility of which, over the pond temperature range of 20.degree. to 100.degree. C. is not too high. For example, ammonium nitrate (NH.sub.4 NO.sub.3) and potassium nitrate (KNO.sub.3), have been considered for a saturated solar pond. The curves of FIG. 1 for these compounds are derived from data from Perry Chemical Engineer's Handbook, 4th Edition. With KNO.sub.3, for example, the high cost and high average solubility over the desired temperature range are such that the KNO.sub.3 for a saturated solar pond would cost in the order of 100 dollars per square meter of pond surface for a typical pond depth of one meter. Such a cost is prohibitively expensive, when it is realized that capital cost in one of the chief contributions to the cost of operating a solar pond. In comparison, with an unsaturated solar pond using M.sub.g Cl.sub.2 or NaCl, the salt would cost in the order of 10 dollars per square meter of pond surface.